Thin film detector and method of manufacture

ABSTRACT

The invention is related to electronic and more specifically to photo-electronic devices manufactured by means of thin film layering techniques. The invention proposes the use of polymer, copolymer, polyamide and similar organic casting materials, in layers 500 to 10,000 angstroms thick, as protective barriers between layers where the processing of one layer would be adversely affected by or would damage an adjacent previously formed layer.

The invention described herein may be manufactured, used, and licensedby the U.S. Government for governmental purposes without the payment ofany royalties thereon.

BACKGROUND OF THE INVENTION

1. FIELD

The invention is related to electronic and photoelectronic devicesmanufactured by means of thin film and monolithic layering techniques.

2. PRIOR ART

Most electronic devices are now made using thin layers of material on asupporting substrate. This has led to the evolution of monolithicsemiconductor devices wherein the active portions of a device reside ina single crystal of silicon, germanium, gallium arsenide, mercurycadmium telluride, and various other similar types of materials. Thesemonolithic devices are now being combined with one another and otherthin film structures to form more exotic devices. For example, an arrayof gallium arsenide photo-detectors which operate in the far-infraredhave been combined with a silicon charged-coupled-device originallydesigned for use with visible light detectors made of silicon. Suchdetectors may be photovoltaic devices, photoconductors or pyroelectricdevices. To interface these devices; with each other, a supportstructure and an optical input element; involves further layeringtechniques. Some of the layering techniques involve electrical isolationor conduction, thermal isolation, differential thermal expansion, andsurface architecture such as crystal lattices and larger irregularities.An object of this invention is to provide a specially prepared thin,flat mechanically stable and/or chemically resistant organic layer, tointerface otherwise incompatible surface layers of microelectronicdevices, and the method of making and using this layer.

SUMMARY OF THE INVENTION

The invention provides a structure and method wherein a barrier layer isformed between two sequential layers of an electronic device which areincompatible. For example, in a thermal or far-infrared photo sensorarray, it would be desirable to deposit a silicon dioxide layer and/or acommon electrode on an aerogel substrate. The latter is an excellentlight weight thermal insulator with good dimensional stability but itssurface is too delicate and porous for electroplating, thermalevaporation, sputtering or wet chemical processing. Due to the nature ofaerogel materials, a dry process must be used to join the barrier layerto the aerogel substrate. The barrier layer is formed separately fromthe substrate and the other layers to achieve a flat structure ofuniform thickness and to avoid chemical and physical damage to thesubstrate or layers. When the completed barrier layer o contacts thesubstrate, it bonds thereto and can be coated by vacuum sputtering orthe other methods mentioned above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood with reference to the drawings wherein:

FIG. 1 shows a mechanical manipulator for the support rings on which theabove mentioned barrier layer may be lifted;

FIG. 2 shows a temperature controlled tray in which the barrier layer isfirst formed;

FIG. 3 shows a ring structure which presents a large contact area to thebarrier layer;

FIG. 4 shows a ring with significantly less contact area than the ringin FIG. 3.

FIG. 5 shows the structure of an electrical device made according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Electronic devices, and particularly microelectronic devices, areusually formed in-situ on some form of substrate. Often the substrate ischosen to facilitate formation of the device, as when the crystallinestructure of the substrate is chosen to match the crystalline structureof a semiconductor used in the active layers of a device. At othertimes, a substrate may be part of another existing structure and mayrequire properties such that direct formation of a device thereon isimpractical. One example is a unique substrate material known as anaerogel. This material was introduced in the early 1930's and usuallyconsists of silica, alumina, zinconia, stannic oxide, tungsten oxide ormixtures thereof. Aerogels are sol-gel derived supercritically driedmaterials with porosities up to 98%. A description of this material andits fabrication is found in the article "AEROGELS--HIGHLY TENUOUS SOLIDSWITH FASCINATING PROPERTIES", by J. Frick, Journal of NoncrystallineSolids, North Holland, Amsterdam), Vol. 100, No. 1-3, pp 169-173.

This material has many interesting qualities as a substrate. It isrigid, but extremely lightweight. It is an excellent thermal insulatorand sound insulator. It is also transparent or translucent to visiblelight and a good absorber of infrared light. It has been suggested as asubstrate for arrays of thermal infrared or pyroelectric detectors toprevent thermal interaction between detectors. These detectors; examplesof which appear in applicant's copending application entitled "Ion BeamEtching of Metal Oxide Ceramics", filed Apr. 30, 1991 and U.S. Pat. No.4,927,771 for "Method of Thermal Isolation of Detector Elements in anUncooled Staring Focal Plane Array" by Donald A. Ferrett, issued May 22,1990; require a uniformly thin common electrode formed by ion-beamsputtering, RF sputtering, magnetron sputtering, etc. These processeswould damage the delicate surface of the aerogel and deposit materialinto its pores producing an electrode with a very non-uniform thickness.

To prevent this, the present invention proposes the use of a barrierlayer between the substrate and the electrode, which must be appliedusing a dry process. Many electronic devices involve this type ofboundary, i.e. a boundary between a nonconducting substrate and aconducting or semiconducting layer. To form the barrier layer, it ispreferred to use such materials as polymers, copolymers, and polyamideswhich can be formed into films between 500 to 10,000 angstroms thick.

FIG. 1 shows a manipulator used to handle such a film in its formation.The manipulator is mounted on a post 10 with a rack 11 attached so thatit can engage a pinion or worm gear (not shown) attached to a table, toraise or lower the beam support 12 which extends parallel to the table.An upper axle member 13 extends through the distal end of the beam fromthe post also parallel to the table. A support rod 14 extends verticallythrough a hole in the axle member and is locked in place by a set screw15. The lower end of the rod terminates in a bearing block 16 whichsupports a lower axle member 17, again, parallel to the table. The loweraxle member is fixed to a ring clamp 18 with a clamping screw 19. Thering clamp has a circular opening slightly larger in diameter than thesupport rings to be used, which are nominally 2.5 inches; although thisis not critical. A friction lever 20 pivoted on the beam presses againstthe vertical rod to level the ring clamp with the table top. A lowertilting lever 21 attached to the ring clamp permits leveling normal tothat of the friction lever.

FIG. 2 shows a thermally controlled tray 22 in which the barrier layersmay be formed. The square tray contains a closed chamber 23 throughwhich warm water is circulated to maintain the desired operatingtemperature. The tray also has a top pan 24 with a flat bottom having awidth C of about 6 inches. The bottom slopes upward to a width B ofabout 12 inches, providing a maximum depth D of about 0.75 inches. Theoverall width A of the tray is about 14 inches and the overall depth Eis about 3 inches. The tray has four equispaced adjustable height legsattached to its lower corners to level the tray with its contents.

FIGS. 3 and 4 show typical ring configurations used in making differentbarrier layers. The ring of FIG. 3 has a maximum diameter F of 2.75inches and minimum diameter of 2.25 inches, thus providing about 2square inches of contact on its bottom surface. The ring of FIG. 4 withmaximum and minimum diameters of 2.5 inches and 2.0 inches and has aminimum radial thickness of 0.15 inch. It presents only 0.57 squareinches of contact on its lower surface. All of the above may be formedfrom stainless steel, aluminum, or plastic. Circular rings are theeasiest to fabricate, but frames of any shape can obviously be used, asdesired. The tray may be lined with Teflon for easy cleaning.

FIG. 5 shows the structure of an electrical device made according topresent invention. The device has a substrate 50 incompatible withvacuum deposition, e.g., an aerogel. On this was placed a precastbarrier layer 51. The barrier layer is covered with a layer 52 ofelectronic material, e.g., a gold electrode. Additional layers can thenbe added in the usual manner to form a complete device. These layers canbe deposited directly on layer 52 or a layer 53 of pyroelectric orsimilar material and can be plated with conductive electrodes 54 and 55,one of which is attached with solder 56 to layer 52.

To form a barrier layer, the following steps are performed:

A. Filling the tray with a liquid subphase with sufficient density andsurface tension to float a mixture of chosen organic barrier materialsand solvents;

B. Circulating water under the tray at a fixed temperature a few degreesor more above room temperature to stabilize the rate of formation of thebarrier layer;

C. Depositing a mixture of barrier material and solvent from a pipetteonto the surface of the liquid subphase;

D. Viewing the spreading mixture to observe interference fringes ofcolors that appear at random over the entire surface of the mixture asit becomes a thin film, the film is relatively uniform in thickness andready for lifting when the interference fringes fade away leaving aclear film;

E. Leveling and lowering a support ring by its supporting structureuntil it lightly contacts the barrier layer;

F. Lifting the barrier layer from the surface of the subphase bysimultaneously lifting and tilting the support rings, e.g. a skilledperson can perform Steps e. and f. by hand using only a ring;

G. Sealing the ring and layer into a vacuum oven, baking the ring andlayer to remove any remaining solvents, filling the oven with an inertgas atmosphere, this is best done in two cycles as indicated in Table IIa short cycle in air and long cycle under vacuum;

H. Removing the ring and barrier layer from the vacuum oven;

I. Deriving the film thickness from the film's reflectivity at normalincidence and the film's index of refraction;

J. Labeling and storing the ring and barrier layer in a dry box;

K. Transferring the film, when needed, from the support ring to a solidsubstrate using a differential pressure such that contact with thesubstrate is made starting in the center of the substrate and moving outto the edges to expel any excess gases trapped between the two;

K1. Step K may also be done entirely by hand at 1 atmosphere or underfull vacuum using a commercially available vacuum manipulator;

L. Sealing the substrate in a vacuum chamber equipped to coat substrateswith one or more materials;

M. Evacuating the chamber;

N. Cooling the substrate and barrier layer to about 2° C.; and

O. Forming at least one thin uniform layer over the exposed broadsurface of the barrier layer.

Table I lists a number of different barrier layer materials, solventstherefor, subphase liquids and subphase additives. This list is by nomeans exhaustive. Hundreds of similar materials will be evident to thoseskilled in the art. Applicant's barrier layers were originally preparedin a Formo Scientific Oven Model 3237, equipped with inert gas input andventing ports, however, to provide greater process control, Steps Gthrough O may be performed continuous under vacuum in a multivestibulechamber. The barrier layer and the substrate would be placed in separatevestibules until the layer is dry and its vestibule evacuated, then bothitems would be located in to the same vestibule. Steps I and J, whichare optional, would probably be omitted. The higher the melting point orsublimation temperature of the material in the barrier layer, the lesslikelihood they will be damaged when applying the first layer of theelectronic device on it. The examples given have melting points of 1-3hundred degrees C. These must be cooled during sputtering or the like,using water near 2° C. As better materials are found, this cooling willnot be necessary. While the physical properties of the layers have beenstressed, the chemical problems that arise between layers can be equallydifficult. Some materials are poisoned by metal ions when addingelectrodes. Some metal alloys contain mercury atoms which migrate easilyinto semiconductors. Some materials simply do not adhere to one another,but will adhere to a barrier material. Since most electrical devicesgenerate heat, it will be beneficial in many cases to protect nearbyelements by introducing an isolating aerogel layer as indicated above.

                                      TABLE I                                     __________________________________________________________________________    BARRIER LAYER FORMULATION EXAMPLES                                            MATERIAL                                                                              GM  SOURCE                                                                              SOLVENT  ML CLASS                                           __________________________________________________________________________    Nitrocellulose                                                                        3.5 Generic                                                                             Ethylacetate                                                                           12.5                                                                             Polymer                                                           Pentyl Acetate                                                                         12.5                                               Polyetherimide                                                                        1.0 Dupont                                                                              1,2,3    10.0                                                                             Polymer                                                           Trichloropropane                                            L.R. 4330                                                                             264 GE    1,2,3    2.0                                                                              Co-Polymer                                                        Trichloropropane                                            L.R. 3320                                                                             132 GE    1,2,3    2.0                                                                              Polymer                                                           Trichloropropane                                            XU-218  400 CIBA  1,2,3    4.0                                                                              Polyimide                                                         Trichloropropane                                            __________________________________________________________________________     Subphase  Deionized water with 0.8% 1Propanol @ 25°                    Subphase Surface Tension: 64.3 dyne/cm @ 18° C.                   

                  TABLE II                                                        ______________________________________                                        BARRIER LAYER DRYING CYCLES                                                           TIME     TEMP      VACUUM                                             CYCLE   (Sec)    °C.                                                                              TORR     BACKFILL                                  ______________________________________                                        I        90       90-100   --       Air                                       II      900      100-110   1.0      Nitrogen                                  ______________________________________                                    

I claim:
 1. A solid state layered electronic device having a thin layerof electronic material deposited over and supported by a substrate ofaerogel material with an upper surface too weak and porous to withstandthe deposition of said layer; including:a thin precast layer of organicmaterial attached to said upper surface said electronic material beingdeposited on said precast layer.
 2. A device according to claim 1,wherein;said electronic material is a thin layer of metal.
 3. A deviceaccording to claim 1, wherein:said electronic material is a thin layerof insulator.
 4. A device according to claim 1, wherein:said electronicmaterial is a thin layer of semiconductor.
 5. A device according toclaim 2, wherein:a thin layer of pyroelectric material is bonded oversaid electronic material.
 6. A device according to claim 3, wherein:athin layer of pyroelectric material is bonded over said electronicmaterial.
 7. A device according to claim 4, wherein:a thin layer ofpyroelectric material is bonded over said electronic material.
 8. Adevice according to claim 1, wherein said electronic materialincludes:at least two sequential thin layers of material chosen from agroup consisting of electrical conductors, insulators, semiconductors,and pyroelectrics.